C40H2各种可能异构体稳定性的理论研究

采用AM1和PM3两种半经验方法,对D_5d对称性的C₄₀及C₄₀H₂所有可能异构体的几何构型进行了非限制对称性全优化,得到51种稳定异构体,在此基础上研究了氢的加成反应规律及本体C₄₀和最稳定及最不稳定C₄₀H₂异构体的红外光谱,讨论了影响C₄₀(D_5d)氢加成异构体稳定性及加成位置选择性的三种主要因素:(1)C₄₀本体几何结构;(2)共轭效应;(3)电荷分布影响.     

Electrospray mass spectrometry in the differentiation of some isomeric trimethylfurocoumarins

Electrospray mass spectrometry (ES-MS) was successfully employed for the structural differentiation of six isomeric trimethylfurocoumarins of possible pharmaceutical interest. Two different approaches were employed. The first was based on MS(n) experiments of MH(+) ions. Although the product ion spectra of MH(+) of the isomers are very similar, the MS(3) spectra of the collisionally generated [MH[bond]CO](+) ions show some characteristic differences. The second approach was based on complexation of the molecules with Li(+), Na(+) and K(+) using ESI-MS of sample solutions containing alkali ions in a 100:1 molar ratio with respect to the analyte. Significant differences were observed in complex production yields, and these were related to the dimension of the alkali ion and to the steric availability of chelating groups in the different isomers.     

Towards the development of a second-order approximation in activity coefficient models based on group contributions

A simple modification of group contribution based models for estimation of liquid phase activity coefficients is proposed. The main feature of this modification is that contribution estimated from the present first-order groups in many instances are found insufficient since the first-order groups do not provide all the important molecular structural information. Therefore, addition of second-order terms, which provide the needed extra molecular structural information, is proposed. The scope of the new approach is demonstrated through the well-known UNIFAC model in terms of improved correlation/prediction capabilities, distinction between isomers and ability to overcome proximity effects.     

New quinoline- and isoquinoline-based multicomponent methods for the synthesis of 1,1(3,3)-dicyanotetrahydrobenzoindolizines

Convenient multicomponent methods for the synthesis of benzannulated dihydroindolizines based on quinoline or isoquinoline, malononitrile, aromatic aldehydes and α-halomethylcarbonyl compounds were developed. Several alternative protocols of using the reactants were studied, starting with separate generation of two most probable intermediates and ending with the four-component condensation of all reactants. The scope of applicability of these methods was found, depending on the initial compounds used. The reaction is highly stereoselective with predominant formation of one of the possible isomers.     

Combined interpretation of chromatographic and mass spectral information in identifying condensation products of carbonyl compounds

The paper considers one of possible approaches to the combined application of chromatographic and mass spectral data, based on the preliminary estimation of retention indices using additive schemes. This algorithm is used to determine structures of isomers formed in the condensation of carbonyl compounds (acetone and cyclohexanone) under basic catalysis.     

Metal-Organic Frameworks for C6 Alkane Separation

The separation of alkane isomers is an important yet challenging process in the petrochemical industry. Being a crucial step to produce premium gasoline components as well as optimum ethylene feed, the current industrial separation by distillation is extremely energy intensive. Adsorptive separation based on zeolite is limited by insufficient adsorption capacity. Metal-organic frameworks (MOFs) hold enormous promise as alternative adsorbents due to their diverse structural tunability and exceptional porosity. Precise control of their pore geometry/dimensions has led to superior performance. In this minireview, we highlight the recent progresses in developing MOFs for the separation of C6 alkane isomers. Representative MOFs are reviewed based on their separation mechanisms. Emphasis is put on the material design rationale for achieving optimal separation capability. Finally, we briefly discuss the existing challenges, possible solutions, and future directions of this important field.     

Separation of isomers of nonylphenol and select nonylphenol polyethoxylates by high-performance liquid chromatography on a graphitic carbon column

p-Nonylphenol (NP) is a ubiquitous degradation product of nonylphenol polyethoxylate (NPE) surfactants and has been reported to be an endocrine disrupter. It is composed of numerous structural isomers resulting from the various branching patterns of the C-9 group. Twenty-two isomers in a technical mix of NP have been identified with high-resolution capillary GC-MS. In most HPLC analyses, nonylphenol elutes as a single, broad peak. In the method described here, HPLC using a graphite carbon column resulted in the resolution of a technical mixture of NP into 12 peaks or groups of isomers. This method was also applied to select NPEs with one to 10 ethoxy units with similar results. Separation was achieved by gradient elution with 1% acetic acid in water and acetonitrile. Elution of individual isomers 4-butylphenol and 4-propylphenol under the same gradient conditions indicate that increased branching of an alkyl group results in shorter retention times than for the less substituted alkyl groups. This method can be used to fractionate NP based on structure and assess the potential for different isomers (or groups of structurally similar isomers) to act as endocrine disrupters.     

A study on the synthesis of structural analogs of bis-indole alkaloid caulerpin: a step-by-step synthesis of a cyclic indole-tetramer

The syntheses of structural isomers of bis-indole alkaloid caulerpin are investigated. Construction of the caulerpin skeleton is based on the Fisher indolization reaction of the appropriate cyclooctane-diones or cyclooctanone. In addition, a step-by-step synthesis of one isomer from possible four cyclic indole-tetramers has first been described.     

High-performance liquid chromatography of non-polar retinoid isomers

Normal-phase high-performance liquid chromatograms of retinal, retinol and retinyl palmitate isomers using n-heptane-tert.-butyl methyl ether as mobile phase are presented. Methods for the synthesis of various isomers of these retinoids are described. This enables one to produce standard chromatograms and to select various isomers for cochromatography and the identification of the various peaks under study. Assignment of the peaks is based on chromatograms published previously in this journal. For the main isomers it is possible to get baseline separation of the commonly occurring isomeric forms in a reasonably short analysis time.     

Determination of ortho-cresyl phosphate isomers of tricresyl phosphate used in aircraft turbine engine oils by gas chromatography and mass spectrometry

Tricresyl phosphate (TCP) is used as an anti-wear additive in aircraft turbine engine oil. Concerns about its toxicity are largely based on the tri-o-cresyl phosphate isomer content. However, the presence of other and more toxic isomers has been previously suggested. In this work, the structural isomers of TCP have been determined by two methods (experimental and semi-theoretical). First, the TCP isomers were separated by gas chromatography (GC) and identified by mass spectrometry (MS). Second, after base cleavage of TCP, GC was used to quantify the cresol precursors. These results were used to calculate the TCP isomer distribution based on the assumption of a statistical distribution of the TCP isomers. The results from the two determinations showed reasonable agreement for three of the four oils studied. The o-cresyl isomers were found to be present almost exclusively as the more toxic mono-o-cresyl isomers in the concentration range 13-150 mg/L. The ability to analyse for the mono-o-cresyl isomers allows the toxicity of TCP to be based on the latter isomers rather than on the less toxic tri-o-cresyl phosphate isomer.     

The generality of architectural isomerism in designer inclusion frameworks.

We describe herein new structural isomers of a lamellar host system based on organodisulfonate "pillars" that connect opposing hydrogen-bonded sheets, consisting of topologically complementary guanidinium (G) ions and sulfonate (S) groups, to generate inclusion cavities between the sheets. These new isomers-zigzag brick, double brick, V-brick, and crisscross bilayer-expand significantly on our earlier report of architectural isomerism displayed by the discrete bilayer and simple brick forms. We demonstrate here that the discrete bilayer-simple brick isomerism, which was limited to several host-guest combinations based on the G(2)(4,4'-biphenyldisulfonate) host and one pair of compounds based on the G(2)(2,6-naphthalenedisulfonate), can be generalized to other organodisulfonate pillars. Furthermore, in many cases the selectivity toward the different framework isomers reflects a rather systematic templating role of the guest molecules and host-guest recognition during assembly of the lattice. We also describe a convenient approach to identifying and classifying the innumerable possible host architectures based upon the pillar projection topologies for the GS sheets and the intersheet connectivities. The discovery of these new architectures reveals a structural versatility for this class of materials that exceeds initial expectations and observations. Each topology produces different connectivities between the sheets in the third dimension that endows each framework isomer with uniquely shaped and sized inclusion cavities, enabling this host system to conform readily to different guests. The unlimited number of architectures available, combined with the inherent conformational softness and structural tunability of these host lattices, suggests a near universality for the GS system with respect to guest inclusion.     

Structural isomers and reactivity for Rh6 and Rh6+

The structure, energetics, and interconversion of isomers of Rh(6) and Rh(6)(+) are studied by using density functional theory with Gaussian basis sets, using guess structures derived from basin-hopping simulations, and obtained by using the Sutton-Chen potential. A large range of spin multiplicities is considered for each isomer. Our calculations suggest two low-lying structures as possible structural isomers: a square bipyramid and a trigonal prism. The reactivity of these two candidate structural isomers with respect to adsorption of nitric oxide is studied via location of reaction transition states and calculation of reaction barriers. Similarities and differences with surface reaction studies are highlighted. These data provide powerful evidence that structural isomerism, and not different spin states, is responsible for the observed biexponential reaction kinetics.     

Synthesis and characterization of bis(di-2-pyridylmethanamine)ruthenium(II)

The complex Ru(dipa)(2)(2+) (dipa = di-2-pyridylmethanamine) has been prepared, yielding approximately a statistical ratio of the meso and rac isomers. The electronic spectra of both isomers show pyridyl pi --> pi transitions in the UV region and MLCT bands in the visible region. The solvent dependence of the spectra provides evidence of hydrogen bond formation between the solvent and the NH(2) site on the ligand. The electrochemical properties of the two isomers are identical; each undergoes a reversible one-electron oxidation in acetonitrile (E(1/2) = 0.933 V vs Ag/AgCl) and in aqueous solution below pH 3 (E(1/2) = 0.786 V vs Ag/AgCl). In aqueous solution above pH 3, one-electron oxidation of the ruthenium center is followed by deprotonation of the ligand NH(2) site yielding a reactive amidoruthenium(III) species. The ruthenium-bound dipa ligand possesses structural constraints that prevent the usual oxidative dehydrogenation reaction, which would yield exclusively the corresponding imine. Instead the amidoruthenium(III) intermediate finds alternative reaction routes leading to multiple products.     

Identification of geometrical isomers using vibrational circular dichroism spectroscopy: a series of mixed-ligand complexes of diamagnetic Co(III) ions

Vibrational circular dichroism (VCD) spectra were measured on the chloroform solutions of a series of mixed-ligand diamagnetic Co(III) complexes, [Co(tfac)(n)(acac)(3-n)] (n = 0-3, tfac = 1,1,1-trifluoro-2,4-pentanedionato; acac = acetylacetonato). Distinct differences were observed in the VCD spectra among the geometrical isomers of the same ligand composition. Such differentiation was hardly possible by their infra-red spectra alone. The structural identification of these isomers was performed in conjunction with DFT calculations.     

Theoretical surface-enhanced Raman spectra study of substituted benzenes II. Density functional theoretical SERS modelling of o-, m-, and p-methoxybenzonitrile

The SERS modelling of o-, m-, and p-methoxybenzonitrile has been performed following the same methodology that in Part I. Optimized structure obtained from DFT calculations in a B3LYP-LANL2DZ level of calculation shows different tilted positions for the isomers under study. From correlations obtained by comparison of Raman and SERS spectra concerning geometrical parameters, frequency shifting, change in band intensity, and force constants is possible to give insight about the different effect of the metal surface on these molecules and the structural reasons of this behaviour. Frontier orbital analysis gives further information and reveals a ligand to metal charge transfer mechanism for all isomers, as well as its relative importance.     

Stoichiometric self-assembly of isomeric, shape-persistent, supramacromolecular bowtie and butterfly structures

Two novel macromolecular constitutional isomers have been self-assembled from previously unreported terpyridine ligands in a three-component system. The terpyridine ligands were synthesized in high yields via a key Suzuki coupling. Restrictions of the possible outcomes for self-assembly ultimately provided optimum conditions for isolation of either a molecular bowtie or its isomeric butterfly motif. These isomers have been characterized by ESI-MS, TWIM-MS, (1)H NMR, and (13)C NMR. Notably, these structural isomers have remarkably different drift times in ion mobility separation, corresponding to different sizes and shapes at high charge states.     

Local aromaticity study of heterocycles using n-center delocalization indices: the role of aromaticity on the relative stability of position isomers

A quantitative study on local aromaticity based on n-center electron delocalization indices, n being the number of atoms in the ring, is performed on a series of heterocycles containing N, O or S. The results indicate that the order of stability within a series of position isomers is not controlled by aromaticity but by other structural factors. Thus, for a certain series of monocycles position isomers (diazoles, triazoles, tetrazoles, diazines, triazines, and tetrazines) the most stable compound is the least aromatic one and vice versa. However, aromaticity controls the stability for series of isomers where these structural factors are similar. For the case of isocompounds, like isobenzopyrrole, isobenzofuran or isobenzothiophene, the large decrease in the aromaticity of the benzene ring with regard to their isomers makes them less stable.     

Separation of single‐base sequential isomers of single‐stranded DNA by capillary electrophoresis and its application in the discrimination of single‐base DNA mutations

Separation of single‐base substitution sequential DNA isomers remains one of the most challenging tasks in DNA separation by capillary electrophoresis. We developed a simple, versatile capillary electrophoresis technique for the separation of single‐base sequential isomers of DNA having the same chain length. This technique is based on charge differences resulting from the different protonation (acid dissociation) properties of the four DNA bases. A mixture of 13 single‐base sequential isomers of 12‐mer single‐stranded DNA was separated by using an electrophoretic buffer solution containing 20 mM phosphoric acid (pH 2.0) and 8 M urea. We demonstrated that our method could separate all possible mutation patterns under identical experimental conditions. In addition, application of our method to the separation of the polymerase chain reaction product of a 68‐mer gene fragment and its single‐base isomers indicates that in combination with the appropriate genomic DNA extraction techniques, the method can detect single‐base gene mutations.     

Structural resolution of carbohydrate positional and structural isomers based on gas-phase ion mobility-mass spectrometry

This report describes the rapid characterization of positional and structural carbohydrate isomers based on structural separations provided by ion mobility-mass spectrometry (IM-MS). Many of the diseases associated with glycoprotein variation can be more effectively treated with earlier detection substantiating the need for high-throughput methodologies for glycan characterization. This remains particularly difficult due to heterogeneity, branching, and large size of carbohydrate moieties which creates the potential for numerous isobaric positional and structural isomers that are difficult to characterize using conventional MS methods. IM-MS provides rapid (μs to ms) structural separations by IM and subsequent identification by MS which presents a means for characterization of positional and structural carbohydrate isomers. To chart the structural variation observed in IM-MS, the ion-neutral collision cross sections for over 300 carbohydrates are reported. This diversity can also be varied through the utility of using different alkali metals to tune separation selectivity via alkali metal-carbohydrate coordination. Furthermore, the advantages of combining either pre- and/or post-IM fragmentation prior to MS analysis is demonstrated for enhanced confidence in carbohydrate identification.     

Selective LC-MS/MS method for the identification of BMAA from its isomers in biological samples

Algal blooms are well-known sources of acute toxic agents that can be lethal to aquatic organisms. However, one such toxin, β-N-methylamino-L-alanine (BMAA) is also believed to cause amyotrophic lateral sclerosis, also known as Lou Gehrig's disease. The detection and identification of BMAA in natural samples were challenging until the recent introduction of reliable methods. However, the issue of potential interference from unknown isomers of BMAA present in samples has not yet been thoroughly investigated. Based on a systematic database search, we generated a list of all theoretical BMAA structural isomers, which was subsequently narrowed down to seven possible interfering compounds for further consideration. The seven possible candidates satisfied the requirements of chemical stability and also shared important structural domains with BMAA. Two of the candidates, 2,4-diaminobutyric acid (DAB) and N-(2-aminoethyl) glycine (AEG) have recently been studied in the context of BMAA. A further isomer, β-amino-N-methyl-alanine (BAMA), has to be considered because it can potentially yield the fragment ion, which is diagnostic for BMAA. Here, we report the synthesis and analysis of BAMA, together with AEG, DAB, and other isomers that are of interest in the separation and detection of BMAA in biological samples by using either high-performance liquid chromatography or ultra-high-performance liquid chromatography coupled with tandem mass spectrometry. We detected for the first time BAMA in blue mussel and oyster samples. This work extends the previously developed liquid chromatography-tandem mass spectrometry platform Spacil et al. (Analyst 135:127, 2010) to allow BMAA isomers to be distinguished, improving the detection and identification of this important amino acid.